Separation units, such as atmospheric distillation units, vacuum distillation units and product strippers, are major processing units in a refinery. Atmospheric or vacuum distillation units separate crude oil into fractions according to boiling point so downstream processing units, such as hydrogen treating or reforming units, will have feedstocks that meet particular specifications. Higher efficiencies and lower costs are achieved if the crude oil separation is accomplished in two steps: first, the total crude oil is fractionated at essentially atmospheric pressure, and second, a bottoms stream of high boiling hydrocarbons, which typically have an initial boiling point below about 800.degree. F. (427.degree. C.), is fed from the atmospheric distillation unit to a second distillation unit operating at a pressure below atmospheric, called a "vacuum" distillation process. The bottoms stream from the atmospheric distillation is also known as topped crude.
The vacuum distillation unit typically separates the bottoms stream coming from the atmospheric unit into various gas oil vapor streams categorized as light gas oil having a boiling point between about 420.degree. and about 610.degree. F. (216.degree.-320.degree. C.), heavy gas oil having a boiling point between about 610.degree. and about 800.degree. F. (320.degree.-427.degree. C.), vacuum gas oil having a boiling range between about 800.degree. and about 1050.degree. F. (427.degree.-566.degree. C.), and vacuum reduced crude having a boiling point above about 1050.degree. F. (566.degree. C.). The vacuum reduced crude is also known as residuum and leaves the vacuum distillation unit as a liquid bottoms stream. Additional information concerning distillation is available in Petroleum Refining Technology and Economics, Gary, J. H. and Handwerk, G. E., pp. 31-51, Marcel Dekker, Inc. (1975).
The vacuum pressure allows the distillation unit to separate the atmospheric unit bottoms into fractions at lower temperatures than if separation were at atmospheric pressure. The high temperatures necessary to vaporize the atmospheric unit bottoms at atmospheric pressure cause thermal cracking to occur, with loss in C5+ yield due to formation of gas, discoloration of the product, and equipment fouling due to coke formation.
In atmospheric or vacuum distillation, lighter hydrocarbons are vaporized and separated from relatively heavier hydrocarbons. Although the heavier hydrocarbons do not vaporize, they may be carried into the lighter hydrocarbons because of entrainment. This is particularly the case within many commercially operated vacuum distillation columns used for processing the bottoms streams from atmospheric columns. The feed stream to the vacuum distillation unit is generally under turbulent conditions and thus the resid is easily entrained in the vapors that are being flashed off from the incoming feed stream.
Entrainment is undesirable because the entrained heavier hydrocarbons are typically contaminated with metals, such as vanadium or nickel, that can poison downstream catalytic processing, such as hydrotreating, hydrocracking, or fluid catalytic cracking, to which portions of the lighter hydrocarbons are typically fed. Most downstream catalytic processes employ fluid beds or fixed beds that contain catalyst materials. For example, a gas oil product, from a vacuum or atmospheric distillation column, may subsequently feed a fluid catalytic cracking unit. If there are metals contained in the feed to a fixed bed hydroconversion process, such as soluble or organometallic compounds, the bed will generally become increasingly plugged with metals as they deposit on the catalyst. These metals deposit themselves in the interstitial space between the catalyst particles, causing the pressure drop to increase. Furthermore, the depositing metals decrease the activity of the catalyst. Therefore, it is desirable to minimize metals, especially nickel and vanadium, that may adversely affect catalyst selectivity and life.
These contaminate metals enter lighter hydrocarbons, such as gas oil, by two routes: (1) by vaporization, because the organometallic compounds have a finite vapor pressure, although their vapor pressure is extremely low and by far the greatest amount of the metallic compounds are in the very heaviest fraction of the bottoms; and (2) by liquid entrained with the gas oil vapors. The elimination of entrainment can only eliminate the metals present in the gas oil via the second route. However, because of the low volatility of the metal compounds, reduction of entrainment should significantly reduce metals content in the lighter hydrocarbons and thus improve performance of downstream catalytic units.
In vacuum distillation, a bottoms stream, separated from crude oil by an atmospheric distillation unit, is fed to a flash zone in the lower portion of the vacuum distillation unit. To reduce entrainment of residiuum from the flash zone, along with the lighter hydrocarbons, such as gas oil, a demister or wire mesh pad is frequently installed at some point between the flash zone and a gas oil draw-off. However, the demister or wire mesh pad is not completely satisfactory for a number of reasons: (1) entrainment in many cases is not found to be significantly reduced; (2) the pads have a tendency to plug with heavy oil and other material; and (3) the pads have a tendency to corrode, with holes resulting from the corrosion.
Methods other than the demister pads have been tried in the past to reduce the entrainment of residuum into the gas oil, but these methods have met with only limited success. Employing a conventional bubble-cap tray above the flash zone causes the vapor to pass through liquid on the bubble-cap tray, thereby allowing vapor to re-entrain liquid droplets. These re-entrained droplets may contain less of the higher boiling components; however, their presence in the vapor stream is deleterious to good fractionation and downstream processing. In addition, the bubble-cap tray exhibits a pressure drop, thus increasing the flash zone pressure required to drive the vapor through the bubble-cap tray. Increased pressure is not desired for the operation of the vacuum distillation column because it necessitates a higher flash zone temperature and prevents a deeper cut distillation.
The bubble-cap could be replaced by a standard chimney tray having a plurality of risers attached to a plate having holes, with a baffle attached to the top of each riser. Chimney trays are available that provide two 90.degree. direction changes--a first 90.degree. direction change when a stream from the riser contacts the baffle, and a second when the stream exits the chimney. These standard chimneys have a lower pressure drop than bubble-caps; however, they allow significant entrainment.
A further problem exists with most de-entrainment devices used in vacuum distillation columns used for processing the bottoms stream from an initial atmospheric distillation column. The bottoms stream is passed into the flash zone of the vacuum tower where a portion of the stream is vaporized and the remaining unvaporized portion --referred to as the residuum or "resid"--collects as a liquid at the bottom of the tower. The vapor stream travels up through the tower, passing through a de-entrainment tray, and then passing through a wash bed where the vapor is contacted with a wash liquid from the tray above. The wash liquid falls onto the de-entrainment tray where it is mixed with the de-entrained resid material. The resid material collected on the de-entrainment tray lowers the value of the wash liquid that is also collected on that tray.
A need exists in the field to design an improved de-entrainment device for separating liquid droplets entrained within a vapor stream, particularly for use in vacuum and atmospheric distillation columns between the flash zone and the separation tray zone. The improved de-entrainment device should provide superior separation of the liquid droplets from the vapor stream with a minimal pressure drop so that when used in vacuum distillation units there is not a significant decrease in the vapor volume or a need to increase the temperature in the flash zone to maintain a given vapor volume. Also, the improved de-entrainment device should function to separate the de-entrained resid material from any wash liquid being used in the tower so as to increase the value of the stream taken off of the de-entrainment tray.